1. Field of the Invention
The present invention relates to a dual tunnel junction sensor without shield layers and, more particularly, to such a dual tunnel junction sensor which has first and second antiparallel (AP) coupled free layer structures which no not require shielding.
2. Description of the Related Art
The heart of a computer is a magnetic disk drive which includes a rotating magnetic disk, a slider that has read and write heads, a suspension arm above the rotating disk and an actuator arm that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disk. The suspension arm biases the slider into contact with the surface of the disk when the disk is not rotating but, when the disk rotates, air is swirled by the rotating disk adjacent an air bearing surface (ABS) of the slider causing the slider to ride on an air bearing a slight distance from the surface of the rotating disk. When the slider rides on the air bearing the write and read heads are employed for writing magnetic impressions to and reading magnetic signal fields from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
An exemplary high performance read head employs a tunnel junction sensor for sensing the magnetic signal fields from the rotating magnetic disk. The sensor includes an insulative tunneling or barrier layer sandwiched between a ferromagnetic pinned layer and a ferromagnetic free layer. An antiferromagnetic pinning layer interfaces the pinned layer for pinning the magnetic moment of the pinned layer 90xc2x0 to an air bearing surface (ABS) wherein the ABS is an exposed surface of the sensor that faces the rotating disk. The tunnel junction sensor is located between ferromagnetic first and second shield layers. First and second leads, which may be the first and second shield layers, are connected to the tunnel junction sensor for conducting a tunneling current (IT) therethrough. The tunneling current (IT) is conducted perpendicular to the major film planes (CPP) of the sensor as contrasted to a spin valve sensor where the tunneling current (IT) is conducted parallel to the major film planes (CIP) of the spin valve sensor. A magnetic moment of the free layer is free to rotate upwardly and downwardly with respect to the ABS from a quiescent or zero bias point position in response to positive and negative magnetic signal fields from the rotating magnetic disk. The quiescent position of the magnetic moment of the free layer, which is parallel to the ABS, is when the tunneling current (IT) is conducted through the sensor without magnetic field signals from the rotating magnetic disk.
When the magnetic moments of the pinned and free layers are parallel with respect to one another the resistance of the tunnel junction sensor to the tunneling current (IT) is at a minimum and when their magnetic moments are antiparallel the resistance of the tunnel junction sensor to the tunneling current (IT) is at a maximum. Changes in resistance of the tunnel junction sensor is a function of cos xcex8, where xcex8 is the angle between the magnetic moments of the pinned and free layers. When the tunneling current (IT) is conducted through the tunnel junction sensor resistance changes, due to signal fields from the rotating magnetic disk, cause potential changes that are detected and processed as playback signals. The sensitivity of the tunnel junction sensor is quantified as magnetoresistive coefficient dr/R where dr is the change in resistance of the tunnel junction sensor from minimum resistance (magnetic moments of free and pinned layers parallel) to maximum resistance (magnetic moments of the free and pinned layers antiparallel) and R is the resistance of the tunnel junction sensor at minimum resistance. The dr/R of a tunnel junction sensor can be on the order of 40% as compared to 10% for a spin valve sensor.
The first and second shield layers may engage the bottom and the top respectively of the tunnel junction sensor so that the first and second shield layers serve as leads for conducting the tunneling current (IT) through the tunnel junction sensor perpendicular to the major planes of the layers of the tunnel junction sensor. The tunnel junction sensor has first and second side surfaces which are normal to the ABS. First and second hard bias layers abut the first and second side surfaces respectively of the tunnel junction sensor for longitudinally biasing the magnetic domains of the free layer. This longitudinal biasing also maintains the magnetic moment of the free layer parallel to the ABS when the read head is in a quiescent condition.
A dual tunnel tunnel junction sensor has been proposed for increasing the magnetoresistive coefficient dr/R by combining resistances of the dual tunnel junction sensor on each side of an antiferromagnetic pinning layer. The dual tunnel junction sensor includes the antiferromagnetic pinning layer which is located between and exchange coupled to each of the first and second pinned layers for pinning magnetic moments of the first and second pinned layers parallel with respect to each other and perpendicular to the ABS. The first and second pinned layers are located between first and second barrier layers and the first and second barrier layers are located between first and second free layer structures. The first and second free layer structures are, in turn, typically located between first and second shield layers for shielding the sensor from all extraneous fields except a signal field from a rotating magnetic disk. Since there are many layers in the dual tunnel junction sensor, the resistance of the sensor is high. Unfortunately, noise is proportional to the resistance which causes a high resistance tunnel junction sensor to produce unwanted noise. There is a strong-felt need to provide dual tunnel junction sensors which have low noise.
The present invention significantly reduces the noise of a dual tunnel junction sensor by eliminating the first and second shield layers. This is accomplished by making the first and second free layer structures first and second antiparallel (AP) coupled structures. The first free layer structure includes a first antiparallel coupling (APC) layer which is located between and interfaces each of the first and second antiparallel (AP) coupled layers and the second free layer structure is a second antiparallel coupling (APC) layer which is located between and interfaces each of the third and fourth AP coupled layers. Each of the first, second, third and fourth AP coupled layers has a magnetic moment. The magnetic moments of the first and fourth AP coupled layers are parallel with respect to each other and the magnetic moments of the second and third AP coupled layers are parallel with respect to each other. Further, the magnetic moments of the first and fourth AP coupled layers are antiparallel with respect to the magnetic moments of the second and third AP coupled layers. This causes the magnetic moments of the second and third AP coupled layers next to the first and second barrier layers respectively to be in-phase so that signal fields from the rotating magnetic disk will be additive on each side of the pinning layer. In one embodiment of the invention the magnetic moment of the first AP coupled layer is greater than the magnetic moment of the second AP coupled layer and the magnetic moment of the third AP coupled layer is greater than the magnetic moment of the fourth AP coupled layer. In the preferred embodiment the magnetic moment of the first AP coupled layer is equal to the magnetic moment of the third AP coupled layer and the magnetic moment of the second AP coupled layer is equal to the magnetic moment of the fourth AP coupled layer.
In the operation of the invention first and second antiparallel signal fields from a perpendicular recorded magnetic disk result in a first signal field rotating the magnetic moment of the first AP coupled layer which, in turn, rotates the magnetic moment of the second AP coupled layer next to the first barrier layer and a second signal field rotates the third AP coupled layer next to the second barrier layer. The rotations of the magnetic moments of the second and third AP coupled layers are in the same direction so that they are in-phase. If the rotation makes these magnetic moments more parallel with respect to the magnetic moments of the pinned layers, the resistance on each side of the pinning layer decreases and if the rotation makes the magnetic moments of the second and third AP coupled layers more antiparallel with respect to the magnetic moments of the pinned layers, the resistance on each side of the pinning layers increases. With the above arrangement, extraneous fields do not impact the sensor because of common mode rejection. An extraneous field will cause the magnetic moments of the second and third AP coupled layers to go in opposite directions which will cause resistances of equal magnitudes but opposite signs on each side of the pinning layer to completely counterbalance each other.
Since shields are not used, the tunnel junction sensor can have a greater stripe height and width so as to further reduce the resistance of the tunnel junction sensor and thereby reduce noise. It should be noted that when shields are used there is a restriction on the height due to lower flux decay length for the shielded case. In the above-described embodiment the read gap is defined by the centers of the first and third AP coupled layers. The present invention is also capable of reading longitudinally recorded magnetic disks. Another advantage of the invention is that the field signals from the rotating magnetic disk can propagate further up into the height of the tunnel junction sensor which increases the signal of the sensor. Another aspect of the invention is to employ electrically nonconductive first and second lead layers in the place of the first and second shield layers. These lead layers can serve the purpose of conducting the tunneling current through the sensor as well as dissipating heat.
An object of the present invention is to provide a low noise dual tunnel junction sensor.
Another object is to provide a low noise tunnel junction sensor which is unshielded, generates less heat and has an increased signal output.
Other objects and attendant advantages of the invention will be appreciated upon reading the following description taken together with the accompanying drawings.